"It has been said," Jim Holt writes in his new book, "that the question Why is there something rather than nothing? is so profound that it would occur only to a metaphysician, yet so simple that it would occur only to a child." I might have reversed the adjectives, but the basic thought that lies behind this – that the child's imagination is, at its core, philosophical and metaphysical, and that the philosopher is the adult who has managed to retain his childhood sense of wonder at the universe – seems to me sound.

And as for this particular question: well, it's a biggie. Why is there stuff – indeed, quite a lot of stuff, as anyone who has walked down Fifth Avenue, visited the Grand Canyon, or simply looked at the night sky, can attest – rather than a whole lot of nothing? (Or would that be a tiny bit of nothing?) Not every question gets, or deserves, its own book, but the question that gives Why Does the World Exist? its title is far too big for any one volume. Holt's book is not meant to be the last word on the matter; it is best seen as an entertaining introduction to a vast range of argument and speculation that would take more lifetimes to master than any of us has at his disposal.

The arguments can get complex but return repeatedly to rest on a couple of basic issues. Here is a question to start with: What, if anything, are we allowed to take for granted when we describe the beginning of the universe? The obvious rejoinder to any proposal that "X caused (or is a reason for) the universe, so the existence of the universe is explained by X" is to say, "Alright, but where did X come from?" (Or, if X is a law or principle, why does X obtain? What makes it true?) This rejoinder is extremely effective when X is, say, God: as Richard Dawkins, among many others, has pointed out, the religious "explanation" of the universe – God made it! – is entirely unsatisfying unless one can also explain who made God. One can hold that God does not need an explanation, of course; but then, why not just say that about the universe itself? As one would expect, there have been attempts to show that God, by his nature, is special and does not need any such explanation; the attempts offered thus far, though, are hopelessly unsatisfying for any questioner not already committed to a religious framework for thought.

But the "So where did X come from?" rejoinder is not only a problem for religious accounts; it also raises issues for any scientifically oriented explanation that aspires to completeness. 'The Higgs boson did it" is as unsatisfying as "God did it" unless we can explain why the Higgs boson and other elementary particles exist at all. (Actually it's not clear that the Higgs boson, if and when its existence is conclusively confirmed, will shed any light on questions of the universe's origins, despite its somehow managing to get itself nicknamed "the God particle.") More seriously, saying that the universe began with the Big Bang doesn't really explain much of anything unless we can explain why the Big Bang happened; if it's just something that came out of nowhere, for no reason, then it can't be counted as a complete explanation. Similarly, a number of physicists have argued that quantum mechanics allows a universe to pop into existence out of something extremely minimal – an energy-packed void, a quantum fluctuation, or something of that ilk. But these accounts still start with something, and so we can still ask where the energy that packed the void came from, or why the laws of quantum mechanics that permit such fluctuations should be as they are. A complete explanation of the universe's existence, as many people understand it, would have to explain those, too.

One might think, then, that a genuinely complete explanation of the world's existence would have to be infinite, since each explaining element would itself need to be explained by something else, something prior. The Harvard philosopher Robert Nozick suggested that the key to ending the regress was to find a principle that did double duty, one that explained the universe's existence and, at the same time, justified itself. His proposal was the so-called "principle of fecundity," which states that "all possible worlds are real." Since the principle of fecundity was itself possible, it would imply its own truth. (If all possible worlds are real, then the possible world or worlds in which the principle of fecundity holds is real.) So it wouldn't need anything else to justify it; it would justify itself. And it would explain why there was stuff in the universe, since stuff is possible, and all possibilities are (somewhere) actualized.

Nozick's proposal is clever, but as Holt observes, it doesn't work. After all, it is not logically possible that all possibilities obtain: some possibilities exclude others. If any possibility that includes the existence of some material object obtains, for instance, then the possibility that there are no material objects anywhere in the multiverse does not obtain. Nozick tried to finesse this by holding that different possibilities obtain "in independent noninteracting realms." But this won't work, because in order to justify itself the principle of fecundity needs to hold across all realms. "Even if all possible planets are realized," Holt explains, "there is no planet where all possibilities are realized. So fecundity is not self-subsuming after all. It's a cruel dilemma for Nozick: either his ultimate explanatory principle leads to contradiction, or it fails to be self-subsuming."

In introducing the idea of possible worlds and treating them as really existing (and not just as logical fictions) we have introduced the concept of the multiverse: a vast, comprehensive reality that includes not only the universe we live in but others as well, perhaps even an infinity of others. Some thinkers – Andrei Linde, for instance – have proposed that our universe is the outgrowth of another. The theory of "cosmic inflation," Linde explains, implies that "the only thing you needed to get a universe like ours started is a hundred-thousandth of a gram of matter.… All the matter in the universe gets created from the negative energy of the gravitational field. So what's to stop us from creating a universe in the lab? We would be like gods!" Thus, "[W]e can't rule out the possibility that our own universe was created by someone in another universe who just felt like doing it."

Of course, as a complete answer to the question of our universe's existence, this faces the same old regress problem: if our universe grew out of another one, where did the other universe come from? (Essentially the same point can be made about Alex Vilenkin's view, which also involves cosmic inflation and allows that from an initial state of nothingness, "a tiny bit of energy-filled vacuum could spontaneously 'tunnel' into existence…[and then] undergo a runaway expansion. In a couple of microseconds it would attain cosmic proportions, issuing in a cascading fireball of light and matter -- the Big Bang!")

No one has yet succeeded, then, in explaining how something could literally come out of nothing: in every case some sort of prior condition needs to be presupposed. Perhaps, though, it is the very idea of "something coming out of nothing" that is at fault. Why not just say, as David Hume suggested, that the universe has always been around, and that the existence of the universe at each moment in time is explained by its existence in the previous moment? Accepting this as the final explanation, of course, involves giving up on the idea that the existence of the universe as a whole can be explained; rather, we would have to accept its existence as a kind of brute fact. But should this bother us?

It doesn't bother me much, to be honest, but it does bother Holt. "Intellectually," he writes, the brute-fact view feels like throwing in the towel. It's one thing to reconcile yourself to a universe with no purpose and no meaning – we've all done that on a dark night of the soul. But a universe without an explanation? That seems an absurdity too far, at least to a reason-seeking species like ourselves. …A world that existed for no reason at all -- an irrational, accidental, "just there" universe -- would be an unnerving one to live in. So, at least, claimed the American philosopher Arthur Lovejoy. In one of his 1933 lectures at Harvard on the "Great Chain of Being," Lovejoy declared that such a world "would have no stability or trustworthiness; uncertainty would infect the whole, anything (except, perhaps, the self-contradictory) might exist and anything might happen, and no one thing would be in itself even more probable than any other."

The first thing to say is that Lovejoy is making at least one error of logic here, by confusing the question of how (and whether) the universe began with the question of what the universe is like. That the universe's existence is irrational (in the sense that it has no ultimate explanation) does not entail that the universe must behave irrationally. The idea that a logical and predictable universe has simply always existed is no more mystifying and no less probable than the idea that a chaotic and fundamentally unpredictable universe might simply always have existed. To think otherwise is to commit the error of thinking that we can use pure reason, unguided by empirical evidence, to determine what a universe is likely to be like.

There is a tendency among many of the thinkers represented here, Holt included, to commit this error – to think, for instance, that we can know just by thinking about it that the universe is more likely to be simple than complex. In Holt's view this is what explains why the existence of the universe is so puzzling. Shouldn't there have been nothing, rather than something, given that nothing is so much simpler than any something? (It's very predictable, for one thing, and takes very little time to describe.) But the idea that the nature of the universe can be known a priori is simply mistaken – a mistake, I can't resist pointing out, that has been well mocked by Douglas Adams, who in The Hitchhiker's Guide to the Galaxy imagined a "stupendous super computer which was so amazingly intelligent that even before the data banks had been connected up it had started from 'I think therefore I am' and got as far as the existence of rice pudding and income tax before anyone managed to turn it off."

Holt's objection to the brute-fact view, then – the view, that is, that the existence of the universe as a whole has no explanation, most likely because it has simply always been around – depends on the questionable view that the existence of the universe requires an explanation: the idea that it is somehow surprising that the universe should exist, and that its nonexistence was more reasonable or more likely. Many people in Holt's book share this view, but there is at least one who doesn't: philosopher of science Adolf Grünbaum, who, Holt writes, "finds the existence of the world utterly unastonishing. And he is utterly convinced that it is rationalfor him to be unastonished." Holt summarizes this thread of Grünbaum's thinking as follows:

Those who profess puzzlement at the existence of a world like ours – one teeming with life and stars and consciousness and dark matter and all kinds of stuff we haven't even discovered yet – seem to have an intellectual prejudice, one that favors the Null World. Nothingness is the natural state of affairs, they implicitly believe, the ontological default option. It is only deviations from nothingness that are mysterious, that require an explanation.

But this last bit, Grünbaum holds – and for my part, I tend to agree – really is nothing more than a metaphysical prejudice: there is no reason to think that an empty universe is any more likely to exist than one that is full of stuff. Indeed, the way we find out what is probable, and what is reasonable to expect, is by looking at how things are – and when we look at how things are, what we find is most decisively not an empty universe! On this view, the existence of stuff, far from being surprising and standing in need of an explanation, is entirely unsurprising. It's the status quo.

This position, then, is precisely the opposite of that held by Oxford theologian Richard Swinburne, who holds that descriptions of reality can be arranged in order of their simplicity.… On a priori grounds, a simple universe is more likely than a complicated one. And the simplest universe of all is the one that contains nothing -- no objects, no properties, no relations. So, prior to the evidence, that is the hypothesis with the greatest probability: the hypothesis that says there is Nothing rather than Something."

This seems deeply erroneous. Choosing the simplest theory that gets correct results (i.e., the simplest one that is adequate to the universe as observed) is a matter of good scientific practice, but it does not reflect an assumption that the universe itself must or even is likely to instantiate any particular degree of simplicity. When we gather evidence – i.e. observe how the world is – we accept it, even when it complicates things (and it often does). We go for the simplest theory that is compatible with the evidence, but the idea that contemporary physics reflects a fundamentally simple reality is utterly wrongheaded, as anyone who has studied contemporary physics will know. Simplicity constrains our theories; it does not constrain what the universe must be like.

Holt is not, ultimately, convinced by Grünbaum's arguments. (Nor is he is convinced by Swinburne's position, which is that the simplest hypothesis consistent with the observable evidence is that God created the universe. This, Holt observes, commits the standard error of theistic explanations, explaining one mystery, the existence of the world, in terms of an even greater one, the existence of God.) In the end – after speaking with luminaries including John Updike (who is genial, charming, and charmingly skeptical) and John Leslie (who holds that the universe exists because it is good and good things have a tendency to exist – a view which, as Holt points out, runs into serious difficulties with the problem of evil) – Holt formulates his own explanatory hypothesis, largely inspired by the work of Oxford philosopher Derek Parfit. Unlike Leslie's view, which seems to imply that reality must be maximally good, and thus perfect, Holt's view, as he formulates it, identifies "the general form that this reality is bound to take" as "that of infinite mediocrity." (This is, at least, in line with what I, at any rate, tend to observe in daily life.) Holt's hypothesis, while it assumes a lot, is interesting and at least a little ingenious, though when he sends it to Parfit he receives only the briefest of replies: "Thanks for the message, which is very interesting. I shall have to think about it carefully…"

"The question Why is there something rather than nothing?sometimes seems vacuous to me," Holt admits at one point. "But in other moods it seems very profound." The latter mood, one guesses, afflicts Holt more frequently. (Why else would "Why Does the World Exist?" exist?) Those who share this mood with Holt, at least from time to time, will almost certainly find this to be an entertaining and thought-provoking book. I must confess, though, that none of the proffered answers to the title question were quite as satisfying to me as the one Holt attributes to the late Sydney Morgenbesser, a professor of philosophy at Columbia University. When a student asked him, "Professor Morgenbesser, why is there something rather than nothing?" Morgenbesser responded, "Oh, even if there was nothing, you still wouldn't be satisfied!"